src/librustc_typeck/coherence/builtin.rs

   1 //! Check properties that are required by built-in traits and set
   2 //! up data structures required by type-checking/codegen.
   3
   4 use rustc::infer::SuppressRegionErrors;
   5 use rustc::infer::outlives::env::OutlivesEnvironment;
   6 use rustc::middle::region;
   7 use rustc::middle::lang_items::UnsizeTraitLangItem;
   8
   9 use rustc::traits::{self, TraitEngine, ObligationCause};
  10 use rustc::ty::{self, Ty, TyCtxt};
  11 use rustc::ty::TypeFoldable;
  12 use rustc::ty::adjustment::CoerceUnsizedInfo;
  13 use rustc::ty::util::CopyImplementationError;
  14 use rustc::infer;
  15
  16 use rustc::hir::def_id::DefId;
  17 use hir::Node;
  18 use rustc::hir::{self, ItemKind};
  19
  20 pub fn check_trait<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, trait_def_id: DefId) {
  21     Checker { tcx, trait_def_id }
  22         .check(tcx.lang_items().drop_trait(), visit_implementation_of_drop)
  23         .check(tcx.lang_items().copy_trait(), visit_implementation_of_copy)
  24         .check(tcx.lang_items().coerce_unsized_trait(), visit_implementation_of_coerce_unsized)
  25         .check(tcx.lang_items().dispatch_from_dyn_trait(),
  26             visit_implementation_of_dispatch_from_dyn);
  27 }
  28
  29 struct Checker<'a, 'tcx: 'a> {
  30     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  31     trait_def_id: DefId
  32 }
  33
  34 impl<'a, 'tcx> Checker<'a, 'tcx> {
  35     fn check<F>(&self, trait_def_id: Option<DefId>, mut f: F) -> &Self
  36         where F: FnMut(TyCtxt<'a, 'tcx, 'tcx>, DefId)
  37     {
  38         if Some(self.trait_def_id) == trait_def_id {
  39             for &impl_id in self.tcx.hir().trait_impls(self.trait_def_id) {
  40                 let impl_def_id = self.tcx.hir().local_def_id(impl_id);
  41                 f(self.tcx, impl_def_id);
  42             }
  43         }
  44         self
  45     }
  46 }
  47
  48 fn visit_implementation_of_drop<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, impl_did: DefId) {
  49     if let ty::Adt(..) = tcx.type_of(impl_did).sty {
  50         /* do nothing */
  51     } else {
  52         // Destructors only work on nominal types.
  53         if let Some(impl_node_id) = tcx.hir().as_local_node_id(impl_did) {
  54             if let Some(Node::Item(item)) = tcx.hir().find(impl_node_id) {
  55                 let span = match item.node {
  56                     ItemKind::Impl(.., ref ty, _) => ty.span,
  57                     _ => item.span,
  58                 };
  59                 struct_span_err!(tcx.sess,
  60                                  span,
  61                                  E0120,
  62                                  "the Drop trait may only be implemented on \
  63                                   structures")
  64                     .span_label(span, "implementing Drop requires a struct")
  65                     .emit();
  66             } else {
  67                 bug!("didn't find impl in ast map");
  68             }
  69         } else {
  70             bug!("found external impl of Drop trait on \
  71                   something other than a struct");
  72         }
  73     }
  74 }
  75
  76 fn visit_implementation_of_copy<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, impl_did: DefId) {
  77     debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
  78
  79     let impl_hir_id = if let Some(n) = tcx.hir().as_local_hir_id(impl_did) {
  80         n
  81     } else {
  82         debug!("visit_implementation_of_copy(): impl not in this crate");
  83         return;
  84     };
  85
  86     let self_type = tcx.type_of(impl_did);
  87     debug!("visit_implementation_of_copy: self_type={:?} (bound)",
  88            self_type);
  89
  90     let span = tcx.hir().span_by_hir_id(impl_hir_id);
  91     let param_env = tcx.param_env(impl_did);
  92     assert!(!self_type.has_escaping_bound_vars());
  93
  94     debug!("visit_implementation_of_copy: self_type={:?} (free)",
  95            self_type);
  96
  97     match param_env.can_type_implement_copy(tcx, self_type) {
  98         Ok(()) => {}
  99         Err(CopyImplementationError::InfrigingFields(fields)) => {
 100             let item = tcx.hir().expect_item_by_hir_id(impl_hir_id);
 101             let span = if let ItemKind::Impl(.., Some(ref tr), _, _) = item.node {
 102                 tr.path.span
 103             } else {
 104                 span
 105             };
 106
 107             let mut err = struct_span_err!(tcx.sess,
 108                                            span,
 109                                            E0204,
 110                                            "the trait `Copy` may not be implemented for this type");
 111             for span in fields.iter().map(|f| tcx.def_span(f.did)) {
 112                 err.span_label(span, "this field does not implement `Copy`");
 113             }
 114             err.emit()
 115         }
 116         Err(CopyImplementationError::NotAnAdt) => {
 117             let item = tcx.hir().expect_item_by_hir_id(impl_hir_id);
 118             let span = if let ItemKind::Impl(.., ref ty, _) = item.node {
 119                 ty.span
 120             } else {
 121                 span
 122             };
 123
 124             struct_span_err!(tcx.sess,
 125                              span,
 126                              E0206,
 127                              "the trait `Copy` may not be implemented for this type")
 128                 .span_label(span, "type is not a structure or enumeration")
 129                 .emit();
 130         }
 131         Err(CopyImplementationError::HasDestructor) => {
 132             struct_span_err!(tcx.sess,
 133                              span,
 134                              E0184,
 135                              "the trait `Copy` may not be implemented for this type; the \
 136                               type has a destructor")
 137                 .span_label(span, "Copy not allowed on types with destructors")
 138                 .emit();
 139         }
 140     }
 141 }
 142
 143 fn visit_implementation_of_coerce_unsized<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, impl_did: DefId) {
 144     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}",
 145            impl_did);
 146
 147     // Just compute this for the side-effects, in particular reporting
 148     // errors; other parts of the code may demand it for the info of
 149     // course.
 150     if impl_did.is_local() {
 151         let span = tcx.def_span(impl_did);
 152         tcx.at(span).coerce_unsized_info(impl_did);
 153     }
 154 }
 155
 156 fn visit_implementation_of_dispatch_from_dyn<'a, 'tcx>(
 157     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 158     impl_did: DefId,
 159 ) {
 160     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}",
 161            impl_did);
 162     if impl_did.is_local() {
 163         let dispatch_from_dyn_trait = tcx.lang_items().dispatch_from_dyn_trait().unwrap();
 164
 165         let impl_node_id = tcx.hir().as_local_node_id(impl_did).unwrap();
 166         let span = tcx.hir().span(impl_node_id);
 167
 168         let source = tcx.type_of(impl_did);
 169         assert!(!source.has_escaping_bound_vars());
 170         let target = {
 171             let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 172             assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 173
 174             trait_ref.substs.type_at(1)
 175         };
 176
 177         debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}",
 178             source,
 179             target);
 180
 181         let param_env = tcx.param_env(impl_did);
 182
 183         let create_err = |msg: &str| {
 184             struct_span_err!(tcx.sess, span, E0378, "{}", msg)
 185         };
 186
 187         tcx.infer_ctxt().enter(|infcx| {
 188             let cause = ObligationCause::misc(span, impl_node_id);
 189
 190             use ty::TyKind::*;
 191             match (&source.sty, &target.sty) {
 192                 (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
 193                     if infcx.at(&cause, param_env).eq(r_a, r_b).is_ok()
 194                     && mutbl_a == *mutbl_b => (),
 195                 (&RawPtr(tm_a), &RawPtr(tm_b))
 196                     if tm_a.mutbl == tm_b.mutbl => (),
 197                 (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
 198                     if def_a.is_struct() && def_b.is_struct() =>
 199                 {
 200                     if def_a != def_b {
 201                         let source_path = tcx.item_path_str(def_a.did);
 202                         let target_path = tcx.item_path_str(def_b.did);
 203
 204                         create_err(
 205                             &format!(
 206                                 "the trait `DispatchFromDyn` may only be implemented \
 207                                 for a coercion between structures with the same \
 208                                 definition; expected `{}`, found `{}`",
 209                                 source_path, target_path,
 210                             )
 211                         ).emit();
 212
 213                         return
 214                     }
 215
 216                     if def_a.repr.c() || def_a.repr.packed() {
 217                         create_err(
 218                             "structs implementing `DispatchFromDyn` may not have \
 219                              `#[repr(packed)]` or `#[repr(C)]`"
 220                         ).emit();
 221                     }
 222
 223                     let fields = &def_a.non_enum_variant().fields;
 224
 225                     let coerced_fields = fields.iter().filter_map(|field| {
 226                         if tcx.type_of(field.did).is_phantom_data() {
 227                             // ignore PhantomData fields
 228                             return None
 229                         }
 230
 231                         let ty_a = field.ty(tcx, substs_a);
 232                         let ty_b = field.ty(tcx, substs_b);
 233                         if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
 234                             if ok.obligations.is_empty() {
 235                                 create_err(
 236                                     "the trait `DispatchFromDyn` may only be implemented \
 237                                      for structs containing the field being coerced, \
 238                                      `PhantomData` fields, and nothing else"
 239                                 ).note(
 240                                     &format!(
 241                                         "extra field `{}` of type `{}` is not allowed",
 242                                         field.ident, ty_a,
 243                                     )
 244                                 ).emit();
 245
 246                                 return None;
 247                             }
 248                         }
 249
 250                         Some(field)
 251                     }).collect::<Vec<_>>();
 252
 253                     if coerced_fields.is_empty() {
 254                         create_err(
 255                             "the trait `DispatchFromDyn` may only be implemented \
 256                             for a coercion between structures with a single field \
 257                             being coerced, none found"
 258                         ).emit();
 259                     } else if coerced_fields.len() > 1 {
 260                         create_err(
 261                             "implementing the `DispatchFromDyn` trait requires multiple coercions",
 262                         ).note(
 263                             "the trait `DispatchFromDyn` may only be implemented \
 264                                 for a coercion between structures with a single field \
 265                                 being coerced"
 266                         ).note(
 267                             &format!(
 268                                 "currently, {} fields need coercions: {}",
 269                                 coerced_fields.len(),
 270                                 coerced_fields.iter().map(|field| {
 271                                     format!("`{}` (`{}` to `{}`)",
 272                                         field.ident,
 273                                         field.ty(tcx, substs_a),
 274                                         field.ty(tcx, substs_b),
 275                                     )
 276                                 }).collect::<Vec<_>>()
 277                                 .join(", ")
 278                             )
 279                         ).emit();
 280                     } else {
 281                         let mut fulfill_cx = TraitEngine::new(infcx.tcx);
 282
 283                         for field in coerced_fields {
 284
 285                             let predicate = tcx.predicate_for_trait_def(
 286                                 param_env,
 287                                 cause.clone(),
 288                                 dispatch_from_dyn_trait,
 289                                 0,
 290                                 field.ty(tcx, substs_a),
 291                                 &[field.ty(tcx, substs_b).into()]
 292                             );
 293
 294                             fulfill_cx.register_predicate_obligation(&infcx, predicate);
 295                         }
 296
 297                         // Check that all transitive obligations are satisfied.
 298                         if let Err(errors) = fulfill_cx.select_all_or_error(&infcx) {
 299                             infcx.report_fulfillment_errors(&errors, None, false);
 300                         }
 301
 302                         // Finally, resolve all regions.
 303                         let region_scope_tree = region::ScopeTree::default();
 304                         let outlives_env = OutlivesEnvironment::new(param_env);
 305                         infcx.resolve_regions_and_report_errors(
 306                             impl_did,
 307                             &region_scope_tree,
 308                             &outlives_env,
 309                             SuppressRegionErrors::default(),
 310                         );
 311                     }
 312                 }
 313                 _ => {
 314                     create_err(
 315                         "the trait `DispatchFromDyn` may only be implemented \
 316                         for a coercion between structures"
 317                     ).emit();
 318                 }
 319             }
 320         })
 321     }
 322 }
 323
 324 pub fn coerce_unsized_info<'a, 'gcx>(gcx: TyCtxt<'a, 'gcx, 'gcx>,
 325                                      impl_did: DefId)
 326                                      -> CoerceUnsizedInfo {
 327     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 328     let coerce_unsized_trait = gcx.lang_items().coerce_unsized_trait().unwrap();
 329
 330     let unsize_trait = gcx.lang_items().require(UnsizeTraitLangItem).unwrap_or_else(|err| {
 331         gcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
 332     });
 333
 334     // this provider should only get invoked for local def-ids
 335     let impl_node_id = gcx.hir().as_local_node_id(impl_did).unwrap_or_else(|| {
 336         bug!("coerce_unsized_info: invoked for non-local def-id {:?}", impl_did)
 337     });
 338
 339     let source = gcx.type_of(impl_did);
 340     let trait_ref = gcx.impl_trait_ref(impl_did).unwrap();
 341     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 342     let target = trait_ref.substs.type_at(1);
 343     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)",
 344            source,
 345            target);
 346
 347     let span = gcx.hir().span(impl_node_id);
 348     let param_env = gcx.param_env(impl_did);
 349     assert!(!source.has_escaping_bound_vars());
 350
 351     let err_info = CoerceUnsizedInfo { custom_kind: None };
 352
 353     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)",
 354            source,
 355            target);
 356
 357     gcx.infer_ctxt().enter(|infcx| {
 358         let cause = ObligationCause::misc(span, impl_node_id);
 359         let check_mutbl = |mt_a: ty::TypeAndMut<'gcx>,
 360                            mt_b: ty::TypeAndMut<'gcx>,
 361                            mk_ptr: &dyn Fn(Ty<'gcx>) -> Ty<'gcx>| {
 362             if (mt_a.mutbl, mt_b.mutbl) == (hir::MutImmutable, hir::MutMutable) {
 363                 infcx.report_mismatched_types(&cause,
 364                                               mk_ptr(mt_b.ty),
 365                                               target,
 366                                               ty::error::TypeError::Mutability)
 367                     .emit();
 368             }
 369             (mt_a.ty, mt_b.ty, unsize_trait, None)
 370         };
 371         let (source, target, trait_def_id, kind) = match (&source.sty, &target.sty) {
 372             (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 373                 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 374                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 375                 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 376                 check_mutbl(mt_a, mt_b, &|ty| gcx.mk_imm_ref(r_b, ty))
 377             }
 378
 379             (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
 380                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 381                 check_mutbl(mt_a, mt_b, &|ty| gcx.mk_imm_ptr(ty))
 382             }
 383
 384             (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => {
 385                 check_mutbl(mt_a, mt_b, &|ty| gcx.mk_imm_ptr(ty))
 386             }
 387
 388             (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b)) if def_a.is_struct() &&
 389                                                                       def_b.is_struct() => {
 390                 if def_a != def_b {
 391                     let source_path = gcx.item_path_str(def_a.did);
 392                     let target_path = gcx.item_path_str(def_b.did);
 393                     span_err!(gcx.sess,
 394                               span,
 395                               E0377,
 396                               "the trait `CoerceUnsized` may only be implemented \
 397                                for a coercion between structures with the same \
 398                                definition; expected `{}`, found `{}`",
 399                               source_path,
 400                               target_path);
 401                     return err_info;
 402                 }
 403
 404                 // Here we are considering a case of converting
 405                 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 406                 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 407                 //
 408                 //     struct Foo<T, U> {
 409                 //         extra: T,
 410                 //         ptr: *mut U,
 411                 //     }
 412                 //
 413                 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 414                 // to `Foo<T, [i32]>`. That impl would look like:
 415                 //
 416                 //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 417                 //
 418                 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 419                 // when this coercion occurs, we would be changing the
 420                 // field `ptr` from a thin pointer of type `*mut [i32;
 421                 // 3]` to a fat pointer of type `*mut [i32]` (with
 422                 // extra data `3`).  **The purpose of this check is to
 423                 // make sure that we know how to do this conversion.**
 424                 //
 425                 // To check if this impl is legal, we would walk down
 426                 // the fields of `Foo` and consider their types with
 427                 // both substitutes. We are looking to find that
 428                 // exactly one (non-phantom) field has changed its
 429                 // type, which we will expect to be the pointer that
 430                 // is becoming fat (we could probably generalize this
 431                 // to multiple thin pointers of the same type becoming
 432                 // fat, but we don't). In this case:
 433                 //
 434                 // - `extra` has type `T` before and type `T` after
 435                 // - `ptr` has type `*mut U` before and type `*mut V` after
 436                 //
 437                 // Since just one field changed, we would then check
 438                 // that `*mut U: CoerceUnsized<*mut V>` is implemented
 439                 // (in other words, that we know how to do this
 440                 // conversion). This will work out because `U:
 441                 // Unsize<V>`, and we have a builtin rule that `*mut
 442                 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 443                 let fields = &def_a.non_enum_variant().fields;
 444                 let diff_fields = fields.iter()
 445                     .enumerate()
 446                     .filter_map(|(i, f)| {
 447                         let (a, b) = (f.ty(gcx, substs_a), f.ty(gcx, substs_b));
 448
 449                         if gcx.type_of(f.did).is_phantom_data() {
 450                             // Ignore PhantomData fields
 451                             return None;
 452                         }
 453
 454                         // Ignore fields that aren't changed; it may
 455                         // be that we could get away with subtyping or
 456                         // something more accepting, but we use
 457                         // equality because we want to be able to
 458                         // perform this check without computing
 459                         // variance where possible. (This is because
 460                         // we may have to evaluate constraint
 461                         // expressions in the course of execution.)
 462                         // See e.g., #41936.
 463                         if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 464                             if ok.obligations.is_empty() {
 465                                 return None;
 466                             }
 467                         }
 468
 469                         // Collect up all fields that were significantly changed
 470                         // i.e., those that contain T in coerce_unsized T -> U
 471                         Some((i, a, b))
 472                     })
 473                     .collect::<Vec<_>>();
 474
 475                 if diff_fields.is_empty() {
 476                     span_err!(gcx.sess,
 477                               span,
 478                               E0374,
 479                               "the trait `CoerceUnsized` may only be implemented \
 480                                for a coercion between structures with one field \
 481                                being coerced, none found");
 482                     return err_info;
 483                 } else if diff_fields.len() > 1 {
 484                     let item = gcx.hir().expect_item(impl_node_id);
 485                     let span = if let ItemKind::Impl(.., Some(ref t), _, _) = item.node {
 486                         t.path.span
 487                     } else {
 488                         gcx.hir().span(impl_node_id)
 489                     };
 490
 491                     let mut err = struct_span_err!(gcx.sess,
 492                                                    span,
 493                                                    E0375,
 494                                                    "implementing the trait \
 495                                                     `CoerceUnsized` requires multiple \
 496                                                     coercions");
 497                     err.note("`CoerceUnsized` may only be implemented for \
 498                               a coercion between structures with one field being coerced");
 499                     err.note(&format!("currently, {} fields need coercions: {}",
 500                                       diff_fields.len(),
 501                                       diff_fields.iter()
 502                                           .map(|&(i, a, b)| {
 503                                               format!("`{}` (`{}` to `{}`)", fields[i].ident, a, b)
 504                                           })
 505                                           .collect::<Vec<_>>()
 506                                           .join(", ")));
 507                     err.span_label(span, "requires multiple coercions");
 508                     err.emit();
 509                     return err_info;
 510                 }
 511
 512                 let (i, a, b) = diff_fields[0];
 513                 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 514                 (a, b, coerce_unsized_trait, Some(kind))
 515             }
 516
 517             _ => {
 518                 span_err!(gcx.sess,
 519                           span,
 520                           E0376,
 521                           "the trait `CoerceUnsized` may only be implemented \
 522                            for a coercion between structures");
 523                 return err_info;
 524             }
 525         };
 526
 527         let mut fulfill_cx = TraitEngine::new(infcx.tcx);
 528
 529         // Register an obligation for `A: Trait<B>`.
 530         let cause = traits::ObligationCause::misc(span, impl_node_id);
 531         let predicate = gcx.predicate_for_trait_def(param_env,
 532                                                     cause,
 533                                                     trait_def_id,
 534                                                     0,
 535                                                     source,
 536                                                     &[target.into()]);
 537         fulfill_cx.register_predicate_obligation(&infcx, predicate);
 538
 539         // Check that all transitive obligations are satisfied.
 540         if let Err(errors) = fulfill_cx.select_all_or_error(&infcx) {
 541             infcx.report_fulfillment_errors(&errors, None, false);
 542         }
 543
 544         // Finally, resolve all regions.
 545         let region_scope_tree = region::ScopeTree::default();
 546         let outlives_env = OutlivesEnvironment::new(param_env);
 547         infcx.resolve_regions_and_report_errors(
 548             impl_did,
 549             &region_scope_tree,
 550             &outlives_env,
 551             SuppressRegionErrors::default(),
 552         );
 553
 554         CoerceUnsizedInfo {
 555             custom_kind: kind
 556         }
 557     })
 558 }